UnityEvent.emit(0) fails

I have a method passing array indexes from React to Unity code, and the code works for all values but 0.

It looks like there's a bug when trying to set a default value :
https://github.com/jeffreylanters/react-unity-webgl/blob/master/source/modules/UnityEvent.js#L13
```js
parameter || ''
```

Falsy values are always replaced by an empty string.

